1. Field of the Invention
The present invention relates to a method of a selective ion etching of a copper thin film which has been deposited on a substrate.
2. Description of the Related Arts
Many of internal wiring for electronic devices, such as semiconductor devices, are formed of a wiring pattern by selectively etching an electrically conductive thin film which has been deposited on a substrate.
Aluminum has been conventionally employed as the material of the electrically conductive thin film. However, for a future fine wiring pattern requiring a high current density therein, copper wiring is required due to its excellent anti-corrosion as well as a low electrical resistance on the high current density condition.
In fabricating the copper wiring pattern a high etching speed is demanded in order to achieve an appropriate productivity. In addition, in order to attain an accurate pattern, an etching selectivity, which allows the use of a thin mask, as well as an etching anisotropy, which allows an accurate shape in the sectional view having no side-etching, are required.
Reactive-ion-etching, referred to hereinafter as RIE, has been an excellent means in forming fine wiring patterns. RIE provides a good anisotropic etching by both of a chemical reaction and physically an ion bombardment. Moreover, RIE has as known well a possibility in that the etching speed can be fast as well as the ratio of the etching speed ratio of the material to be etched to the etching speed of the mask material can be enhanced. However, only a little has been known about a RIE method that allows a copper thin film to be etched selectively as well as anisotropically.
There was, for example, reported a RIE of the copper thin film which used plasma of a gas mixture of CCl.sub.4 and Ar, by G. C. Sehwart and P. M. Sehlble as "Reactive Ion Etching of Copper Films", on Journal of Electrochemical Society 130, p. 1777, (1983). Here was used a silicon oxide film as a etching mask. Etching speed of this RIE using the gas mixture of CCl.sub.4 and Ar was small. Therefore, it was necessary to increase the electric power density to excite the plasma in order to hasten the etching speed. Under the condition of the high electric power density, the etching speed ratio of the copper film to the silicon oxide mask, in other words an etching selectivity, was not so large, for example, the etching speed ratio was only about 1 to 1.5. Therefore, a thick etching mask had to be used, resulting in a deterioration of the wiring pattern accuracy. Moreover, under the condition of the slow etching speed, it was difficult to etch a narrow ditch because new copper is adhered onto the sidewall of the etched ditch.
Another method to etch a copper film was reported by Kazuhide Ohno, Masaaki Sato, and Yoshinobu Arita in the title of "High Rate Reactive Ion Etching of Copper Films in SiCl.sub.4, N.sub.2, Cl.sub.2 and NH.sub.3 Mixture" on p. 215-218 (1990) of Extended Abstract of 22nd (1990 International) Conference on Solid State Devices and Materials, held in Sendai. It is reported there that the etching speed of the copper thin film in this RIE was 100 nm/minute, which was much faster than the etching speed (20 nm/minute) of the case where a gas mixture merely of the silicon tetrachloride and nitrogen was used as the etching gas. However, in this method of the RIE of the copper thin film it was difficult to achieve an etching speed more than this without deterioration in the shape accuracy of the etched copper thin film pattern.
On the other hand, a RIE of a thin aluminum alloy film containing 1% copper employing a gas mixture of SiCl.sub.4, N.sub.2, and CCl.sub.4 was disclosed in Japanese Provisional Patent Publication Sho 63-181429 (1988). In the preferred embodiment disclosed in this publication, it was reported that the etching speed was 750 nm/minute, the etching speed ratio of a thin 1% Al-1% Si--Cu alloy film to the SiO.sub.2 film was 12, the decrease in the wiring width was 0.1 .mu.m, and the etching was accomplished without leaving copper residue on the surface of the substrate surface, under the condition that the resist pattern was used as a mask, the electrode on which the substrate was placed was water-cooled, and the gas flow ratio of reaction gas of SiCl.sub.4, N.sub.2, and CCl.sub.4 was 55%: 15%: 30%.
Thus, in the prior art RIE method for thin copper film there were disadvantageous problems in that the etching speed was low, it was apt to cause a side-etching, and accordingly, it was impossible to reproduce an accurate pattern due to the inadequate etching selectivity between the mask and the copper film.